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Leti’s CoolCube 3D Transistor Stacking Improves with Qualcomm Help

Wednesday, April 27th, 2016

By Ed Korczynski, Sr. Technical Editor

As previously covered by Solid State Technology CEA-Leti in France has been developing monolithic transistor stacking based on laser re-crystallization of active silicon in upper layers called “CoolCube” (TM). Leading mobile chip supplier Qualcomm has been working with Leti on CoolCube R&D since late 2013, and based on preliminary results have opted to continue collaborating with the goal of building a complete ecosystem that takes the technology from design to fabrication.

“The Qualcomm Technologies and Leti teams have demonstrated the potential of this technology for designing and fabricating high-density and high-performance chips for mobile devices,” said Karim Arabi, vice president of engineering, Qualcomm Technologies, Inc. “We are optimistic that this technology could address some of the technology scaling issues and this is why we are extending our collaboration with Leti.” As part of the collaboration, Qualcomm Technologies and Leti are sharing the technology through flexible, multi-party collaboration programs to accelerate adoption.

Olivier Faynot, micro-electronic component section manager of CEA-Leti, in an exclusive interview with Solid State Technology and SemiMD explained, “Today we have a strong focus on CMOS over CMOS integration, and this is the primary integration that we are pushing. What we see today is the integration of NMOS over PMOS is interesting and suitable for new material incorporation such as III-V and germanium.”

Table: Critical thermal budget steps summary in a planar FDSOI integration and CoolCube process for top FET in 3DVLSI. (Source: VLSI Symposium 2015)

The Table shows that CMOS over CMOS integration has met transistor performance goals with low-temperature processes, such that the top transistors have at least 90% of the performance compared to the bottom. Faynot says that recent results for transistors are meeting specification, while there is still work to be done on inter-tier metal connections. For advanced ICs there is a lot of interconnect routing congestion around the contacts and the metal-1 level, so inter-tier connection (formerly termed the more generic “local interconnect”) levels are needed to route some gates at the bottom level for connection to the top level.

“The main focus now is on the thermal budget for the integration of the inter-tier level,” explained Faynot. “To do this, we are not just working on the processing but also working closely with the designers. For example, depending on the material chosen for the metal inter-tier there will be different limits on the metal link lengths.” Tungsten is relatively more stable than copper, but with higher electrical resistance for inherently lower limits on line lengths. Additional details on such process-design co-dependencies will be disclosed during the 2016 VLSI Technology Symposium, chaired by Raj Jammy.

When the industry decides to integrate III-V and Ge alternate-channel materials in CMOS, the different processing conditions for each should make NMOS over PMOS CoolCube a relatively easy performance extension. “Three-fives and germanium are basically materials with low thermal budgets, so they would be most compatible with CoolCube processing,” reminded Faynot. “To me, this kind of technology would be very interesting for mobile applications, because it would achieve a circuit where the length of the wires would be shortened. We would expect to save in area, and have less of a trade-off between power-consumption and speed.”

“This is a new wave that CoolCube is creating and it has been possible thanks to the interest and support of Qualcomm Technologies, which is pushing the technological development in a good direction and sending a strong signal to the microelectronics community,” said Leti CEO Marie Semeria. “Together, we aim to build a complete ecosystem with foundries, equipment suppliers, and EDA and design houses to assemble all the pieces of the puzzle and move the technology into the product-qualification phase.”


Solid State Watch: November 13-19, 2015

Friday, November 20th, 2015
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Click here to read more acquisition news.

Solid State Watch: July 10-16, 2015

Friday, July 17th, 2015
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Monolithic 3D processing using non-equilibrium RTP

Friday, April 17th, 2015


By Ed Korczynski, Senior Technical Editor, Solid State Technology

Slightly more than one year after Qualcomm Technologies announced that it was assessing CEA-Leti’s monolithic 3D (M3D) transistor stacking technology, Qualcomm has now announced that M3D will be used instead of through-silicon vias (TSV) in the company’s next generation of cellphone handset chips. Since Qualcomm had also been a leading industrial proponent of TSV over the last few years while participating in the imec R&D consortium, this endorsement of M3D is particularly relevant.

Leti’s approach to 3D stacking of transistors starts with a conventionally built and locally-interconnected bottom layer of transistors, which are then covered with a top layer of transistors built using relatively low-temperature processes branded as “CoolCube.” Figure 1 shows a simplified cross-sectional schematic of a CoolCube stack of transistors and interconnects. CoolCube M3D does not transfer a layer of built devices as in the approach using TSV, but instead transfers just a nm-thin layer of homogenous semiconducting material for subsequent device processing.

Fig. 1: Simplified cross-sectional rendering of Monolithic 3D (M3D) transistor stacks, with critical process integration challenges indicated. (Source: CEA-Leti)

The reason that completed transistors are not transferred in the first place is because of intrinsic alignment issues, which are eliminated when transistors are instead fabricated on the same wafer. “We have lots of data to prove that alignment precision is as good as can be seen in 2D lithography, typically 3nm,” explained Maud Vinet, Leti’s advanced CMOS laboratory manager in an exclusive interview with SST.

As discussed in a blog post online at Semiconductor Manufacturing and Design ( last year by Leti researchers, the M3D approach consists of sequentially processing:

  • processing a bottom MOS transistor layer with local interconnects,
  • bonding a wafer substrate to the bottom transistor layer,
  • chemical-mechanical planarization (CMP) and SPE of the top layer,
  • processing the top device layer,
  • forming metal vias between the two device layers as interconnects, and
  • standard copper/low-k multi-level interconnect formation.

To transfer a layer of silicon for the top layer of transistors, a cleave-layer is needed within the bulk silicon or else time and money would be wasted in grinding away >95% of the silicon bulk from the backside. For CMOS:CMOS M3D thin silicon-on-insulator (SOI) is the transferred top layer, a logical extension of work done by Leti for decades. The heavy dose ion-implantation that creates the cleave-layer leaves defects in crystalline silicon which require excessively high temperatures to anneal away. Leti’s trick to overcome this thermal-budget issue is to use pre-amorphizing implants (PAI) to completely dis-order the silicon before transfer and then solid-phase epitaxy (SPE) post-transfer to grow device-grade single-crystal silicon at ~500°C.

Since neither aluminum nor copper interconnects can withstand this temperature range, the interconnects for the bottom layer of transistors need to be tungsten wires with the highest melting point of any metal but somewhat worse electrical resistance (R). Protection for the lower wires cannot use low-k dielectrics, but must use relatively higher capacitance (C) oxides. However, the increased RC delay in the lower interconnects is more than offset by the orders-of-magnitude reduction in interconnect lengths due to vertical stacking.

M3D Roadmaps

Leti shows data that M3D transistor stacking can provide immediate benefit to industry by combining two 28nm-node CMOS layers instead of trying to design and manufacture a single 14nm-node CMOS layer:  area gain 55%, performance gain 23%, and power gain 12%. With cost/transistor now expected to increase with sequential nodes, M3D thus provides a way to reduce cost and risk when developing new ICs.

For the industry to use M3D, there are some unique new unit-processes that will need to ramp into high-volume manufacturing (HVM) to ensure profitable line yield. As presented by C. Fenouillet-Beranger et al. from Leti and ST (paper 27.5) at IEDM2014 in San Francisco, “New Insights on Bottom Layer Thermal Stability and Laser Annealing Promises for High Performance 3D Monolithic Integration,” due to stability improvement in bottom transistors found through the use of doping nickel-silicide with a noble metal such as platinum, the top MOSFET processing temperature could be relaxed up to 500°C. Laser RTP annealing then allows for the activation of top MOSFETs junctions, which have been characterized morphologically and electrically as promising for high performance ICs.

Figure 2 shows the new unit-processes at <=500°C that need to be developed for top transistor formation:

*   Gate-oxide formation,

*   Dopant activation,

*   Epitaxy, and

*   Spacer deposition.

Fig. 2: Thermal processing ranges for process modules need to be below ~500°C for the top devices in M3D stacks to prevent degradation of the bottom layer. (Source: CEA-Leti)

After the above unit-processes have been integrated into high-yielding process modules for CMOS:CMOS stacking, heterogeneous integration of different types of devices are on the roadmap for M3D. Leti has already shown proof-of-concept for processes that integrate new IC functionalities into future M3D stacks:

1)       CMOS:CMOS,

2)       PMOS:NMOS,

3)       III-V:Ge, and

4)       MEMS/NEMS:CMOS.

Thomas Ernst, senior scientist, Electron Nanodevice Architectures, Leti, commented to SST, “Any application that will need a ‘pixelated’ device architecture would likely use M3D. In addition, this approach will work well for integrating new channel materials such as III-V’s and germanium, and any materials that can be deposited at relatively low temperatures such as the active layers in gas-sensors or resistive-memory cells.”

Non-Equilibrium Thermal Processing

Though the use of an oxide barrier between the active device layers provides significant thermal protection to the bottom layer of devices during top-layer fabrication, the thermal processes of the latter  cannot be run at equilibrium. “One way of controlling the thermal budget is to use what we sometimes call the crème brûlée approach to only heat the very top surface while keeping the inside cool,” explained Vinet. “Everyone knows that you want a nice crispy top surface with cool custard beneath.” Using a laser with a short wavelength prevents penetration into lower layers such that essentially all of the energy is absorbed in the surface layer in a manner that can be considered as adiabatic.

Applied Materials has been a supplier-partner with Leti in developing M3D, and the company provided responses from executive technologists to queries from SST about the general industry trend to controlling short pulses of light for thermal processing. “Laser non-equilibrium heating is enabling technology for 3D devices,” affirmed Steve Moffatt, chief technology officer, Front End Products, Applied Materials. “The idea is to heat the top layer and not the layers below. To achieve very shallow adiabatic heating the toolset needs to ramp up in less than 100 nsec. In order to get strong absorption in the top surface, shorter wavelengths are useful, less than 800 nm. Laser non-equilibrium heating in this regime can be a critical process for building monolithic 3D structures for SOC and logic devices.”

Of course, with ultra-shallow junctions (USJ) and atomic-scale gate-stacks already in use for CMOS transistors at the 22nm-node, non-equilibrium thermal processing has already been used in leading fabs. “Gate dielectric, gate metal, and contact treatments are areas where we have seen non-equilibrium anneals slowly taking the place of conventional RTP,” clarified Abhilash Mayur, senior director, Front End Products, Applied Materials. “For approximate percentages, I would say about 25 percent of thermal processing for logic at the 22nm-node is non-equilibrium, and seen to be heading toward 50 percent at the 10nm-node or lower.”

Mayur further explained some of the trade-offs in working on the leading-edge of thermal processing for demanding HVM customers. Pulse-times are in the tens of nsec, with longer pulses tending to allow the heat to diffuse deeper and adversely alter the lower layers, and with shorter pulses tending to induce surface damage or ablation. “Our roadmap is to ensure flexibility in the pulse shape to tailor the heat flow to the specific application,” said Mayur.

Now that Qualcomm has endorsed CoolCube M3D as a preferred approach to CMOS:CMOS transistor stacking in the near-term, we may assume that R&D in novel unit-processes has mostly concluded. Presumably there are pilot lots of wafers now being run through commercial foundries to fine-tune M3D integration. With a roadmap for long-term heterogeneous integration that seems both low-cost and low-risk, M3D using non-equilibrium RTP will likely be an important way to integrate new functionalities into future ICs.

Solid State Watch: November 25-December 4, 2014

Monday, December 8th, 2014
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Solid State Watch: SEMICON Europa Special Edition

Thursday, October 16th, 2014
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Blog review June 30, 2014

Monday, June 30th, 2014

Pete Singer blogs that at The ConFab last week, IBM’s Gary Patton gave us three reasons to be very positive about the future of the semiconductor industry: an explosion of applications, the rise of big data and the need to analyze all that data.

Tony Chao of Applied Materials writes that Applied Ventures will be participating in the second-annual Silicon Innovation Forum (SIF) held in conjunction with SEMICON West 2014 in San Francisco on Tuesday, July 8. The forum is designed to bring new and emerging innovators together with the semiconductor industry’s top strategic investors and venture capitalists (VCs), in order to enable closer collaboration and showcase the next generation of entrepreneurs in microelectronics.

Adele Hars of ASN recently caught up again with Laurent Malier, CEO of CEA-Leti to get his take on the ST-Samsung news. Malier said that CEA-Let has been heavily investing in FD-SOI technology, committing critical scientific and technological support at each phase of FD-SOI development.

Phil Garrou blogs that last week at the 2014 ISC (International Supercomputing Conference) it was announced that the Intel Xenon Phi processor “Knights Landing” would debut in 2015. It will be manufactured by Intel using 14nm FinFET process technology and will include up to 72 processor cores that can work on up to four threads per core.

ST licenses 28nm FD-SOI to Samsung

Friday, May 16th, 2014

By Ed Korczynski, Sr. Technical Editor, SST/SemiMD

On May 14, 2014 it was announced that STMicroelectronics and Samsung Electronics signed an agreement on 28nm Fully Depleted Silicon-on-Insulator (FD-SOI) technology for multi-source manufacturing collaboration. The agreement includes ST’s fully developed process technology and design enablement ecosystem from its 300mm facility in Crolles, France. The Samsung 28nm FD-SOI process will be qualified in early 2015 for volume production.

“Building upon the existing solid relationship between ST and Samsung within the framework of the International Semiconductor Development Alliance, this 28nm FD-SOI agreement expands the ecosystem and augments fab capacity for ST and the entire electronics industry,” said Jean-Marc Chery, COO, STMicroelectronics. “We foresee further expansion of the 28nm FD-SOI ecosystem, to include the leading EDA and IP suppliers, which will enrich the IP catalog available for 28nm FD-SOI.”

According to Handel Jones, founder and CEO of International Business Strategies Inc. (IBS), “The 28nm node will be long-lived; we expect it to represent approximately 4.3 million wafers in the 2017 timeframe, and FD-SOI could capture at least 25 percent of this market.”

Table 1 shows IBS data estimating costs for different 28nm fab process technologies.

“We are pleased to announce this 28nm FD-SOI collaboration with ST. This is an ideal solution for customers looking for extra performance and power efficiency at the 28nm node without having to migrate to 20nm,” said Dr. Seh-Woong Jeong, executive vice president of System LSI Business, Samsung Electronics. “28nm process technology is a highly productive process technology and expected to have a long life span based on well-established manufacturing capabilities.”

In June 2012, ST announced that GLOBALFOUNDRIES had joined the FD-SOI party for the 28nm and 20nm nodes. However, though the name has since changed from “20nm” to “14nm” (Table 2), work continues nonetheless with GLOBALFOUNDRIES on 14nm FD-SOI with prototyping and IP validation vehicles planned to run by the end of this year. Samsung has so far only licensed the 28nm node technology from ST. A representative of GLOBALFOUNDRIES reached for comment on this news expressed welcome to Samsung as an additional supplier in the FD-SOI ecosystem.

“Leti continues its development of further generations and our technology and design results show great promise for the 14nm and 10nm nodes,” said Laurent Malier, CEO of CEA-Leti (Laboratory for Electronics and Information Technology). Leti and ST are not against finFET technology, but sees it as complementary to SOI. In fact the ecosystem plans to add finFETs to the FD-SOI platform for the 10nm node, at which point Taiwanese foundry UMC plans to join.

FD-SOI Substrate Technology

Soitec, a world leader in generating and manufacturing revolutionary semiconductor materials for the electronics and energy industries, supplies most of the world’s SOI wafers. Paul Boudre, COO of Soitec, commented, “Our FD-SOI wafers represent an incredible technology achievement, resulting from over 10 years of continuous research and high-volume manufacturing expertise. With our two fabs and our licensing strategy, the supply chain is in place and we are very excited by this opportunity to provide the semiconductor industry with our smart substrates in high volume to enable widespread deployment of FD-SOI technology.”

Soitec’s R&D of ultra-thin SOI was partly funded and facilitated by the major French program called “Investments for the Future.” Soitec has collaborated with CEA-Leti on process evolution and characterization, with IBM Microelectronics for device validation and collaboration, and with STMicroelectronics to industrialize and demonstrate the first products.

Boudre, in an exclusive interview with SST/SemiMD, explained, “For 28nm node processing we use a 25+-1nm buried oxide layer, which is reduced in thickness to 20+-1nm when going to the 14nm node and we don’t see any differences in the substrate production. However, for the 10nm node the buried oxide layer needs to be 15nm thin, and we will need some new process steps to be able to embed nMOS strain into substrates.”


Blog review April 14, 2014

Monday, April 14th, 2014

The increased performance and the rapid shift from traditional handsets to consumer computing device post a number of manufacturing and supply chain challenges for fabless chip makers. Dr. Roawen Chen of Qualcomm says the scale of the challenges also creates an “extreme stress” for the existing foundry/fabless model to defend its excellence in this dynamic landscape. In a keynote talk at The ConFab, titled “what’s on our mind?” Dr. Chen will deliberate on a number of headwinds and opportunities.

Jean-Eric Michallet, Hughes Metras and Perrine Batude of CEA-Leti describe how the research group has already demonstrated the successful stacking of Si CMOS on Si CMOS, achieving benchmark performance for both layers of transistors. The main process challenge is to develop a sufficiently low-temperature process for the top transistor layer to limit the impact on the lower transistor layers.

Phil Garrou continues his analysis of the IMAPS Device Packaging Conference with a review of the keynote by AMD’s Bryan Black, titled“Die Stacking and High Bandwidth Memory.” Black stated that “…while die stacking is catching on in FPGAs, Power Devices, and MEMs, there is nothing in mainstream computing CPUs, GPUs, and APUs …HBM Stacked DRAM will change this!” Garrou also reviews the newly announced STATSChipPAC FlexLine, which uses eWLB technology to dice and reconstitute incoming wafers of various sizes to a standard size, which results in wafer level packaging equipment becoming independent of incoming silicon wafer size.

Karen Savala, president, SEMI Americas, blogs about the sustainable manufacturing imperative, noting that sustainability is increasingly considered a differentiating factor in global competitiveness relative to the technologies and products being provided. In conjunction with SEMICON West and INTERSOLAR North America, SEMI is organizing a four-day Sustainable Manufacturing Forum to share information about the latest technologies, products, and management approaches that promote sustainable manufacturing.

The Week in Review: March 28, 2014

Friday, March 28th, 2014

Altera Corporation and Intel Corporation announced their collaboration on the development of multi-die devices that leverage Intel’s package and assembly capabilities and Altera’s leading-edge programmable logic technology. The collaboration is an extension of the foundry relationship between Altera and Intel, in which Intel is manufacturing Altera’s Stratix 10 FPGAs and SoCs using the 14nm Tri-Gate process. Altera’s work with Intel will enable the development of multi-die devices that efficiently integrates monolithic 14nm Stratix 10 FPGAs and SoCs with other advanced components, which may include DRAM, SRAM, ASICs, processors and analog components, in a single package.

Samsung introduced a new lineup of flip chip LED packages and modules offering enhanced design flexibility and a high degree of reliability. The new offerings, for use in leading-edge LED lighting such as LED bulbs, MR/PAR and downlights, will be available in the market during the second quarter of this year. Samsung’s new flip chip (FC) LED package and flip chip on module (FCOM) solutions feature highly efficient and versatile LED structures, created by flipping over blue LED chips and adhering phosphor film to each of them. Unlike conventional LED packages that dispense phosphor and then place a plastic mold over each chip, Samsung’s FC package technology can produce LED packages down to a chip-scale size without any mold, enabling more compact lighting fixture designs.

eInfochips, a semiconductor and product engineering company, this week launched design services for chips based on 16nm geometry. The comprehensive suite of services includes Netlist to GDSII, Sign-off, and Design for Testability. eInfochips is one of the few engineering services companies in the world capable of delivering 16nm chip designs which reduce a chip’s power consumption by half, while improving performance by one-third over 28nm technology.

SEMATECH announced this week that Particle Measuring Systems has joined SEMATECH to advance the development of nanoscale particle removal processes and cleaning technologies for next-generation wafers and devices. This collaboration will address many of the profound changes taking place in the semiconductor industry that are impacting fundamental aspects of process and equipment design, including integration of new materials and process technology for sub-20nm node manufacturing, next-generation lithography requirements.

CEA-Leti will demonstrate its new prototype for wireless high data rate Li-Fi (light fidelity) transmission at Light + Building 2014 in Frankfurt, Germany, March 30-April 4. The technology employs the high-frequency modulation capabilities of light-emitting diode (LED) engines used in commercial lighting. It achieves throughputs of up to 10Mb/s at a range of three meters, suitable for HD video streaming or Internet browsing, using light power of less than 1,000 lumens and with direct or even indirect lighting. With this first proof of concept and its expertise in RF communications, Leti forecasts data transmission rates in excess of 100Mb/s with traditional lighting based on LED lamps using this technology approach and without altering the high-performance lighting characteristics.

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